Method for device-to-device (D2D) operation performed by terminal in wireless communication system and terminal using the method

ABSTRACT

A method for device-to-device (D2D) operation performed by a terminal in a radio resource control (RRC) idle state in a wireless communication system, and a terminal using the method are provided. The method comprises: determining whether a serving cell of the terminal is a suitable cell; determining whether the serving sell provides resource pool information; and, if the serving cell is a suitable cell, and the serving cell provides the resource pool information, transmitting a D2D signal within resources indicated by the resource pool information.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/125,550, filed on Sep. 12, 2016, now U.S. Pat. No. 9,699,826, whichis the National Stage filing under 35 U.S.C. 371 of InternationalApplication No. PCT/KR2015/002843, filed on Mar. 23, 2015, which claimsthe benefit of U.S. Provisional Application No. 61/969,001, filed onMar. 21, 2014, the contents of which are all hereby incorporated byreference herein in their entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to wireless communication, and moreparticularly, to a method for a D2D operation performed by a terminal ina wireless communication system and a terminal using the method.

Related Art

In International Telecommunication Union Radio communication sector(ITU-R), a standardization task for International MobileTelecommunication (IMT)-Advanced, that is, the next-generation mobilecommunication system since the third generation, is in progress.IMT-Advanced sets its goal to support Internet Protocol (IP)-basedmultimedia services at a data transfer rate of 1 Gbps in the stop andslow-speed moving state and of 100 Mbps in the fast-speed moving state.

For example, 3rd Generation Partnership Project (3GPP) is a systemstandard to satisfy the requirements of IMT-Advanced and is preparingfor LTE-Advanced improved from Long Term Evolution (LTE) based onOrthogonal Frequency Division Multiple Access (OFDMA)/SingleCarrier-Frequency Division Multiple Access (SC-FDMA) transmissionschemes. LTE-Advanced is one of strong candidates for IMT-Advanced.

There is a growing interest in a Device-to-Device (D22) technology inwhich devices perform direct communication. In particular, D2D has beenin the spotlight as a communication technology for a public safetynetwork. A commercial communication network is rapidly changing to LTE,but the current public safety network is basically based on the 2Gtechnology in terms of a collision problem with existing communicationstandards and a cost. Such a technology gap and a need for improvedservices are leading to efforts to improve the public safety network.

The public safety network has higher service requirements (reliabilityand security) than the commercial communication network. In particular,if coverage of cellular communication is not affected or available, thepublic safety network also requires direct communication betweendevices, that is, D2D operation.

D2D operation may have various advantages in that it is communicationbetween devices in proximity. For example, D2D UE has a high transferrate and a low delay and may perform data communication. Furthermore, inD2D operation, traffic concentrated on a base station can bedistributed. If D2D UE plays the role of a relay, it may also play therole of extending coverage of a base station.

Meanwhile, when a terminal camps on a specific cell in a radio resourcecontrol (RRC) idle state, the standard did not definitely prescribe bywhich resource and under which condition a D2D signal is transmitted.Definitely prescribing the standard is required for reliability of a D2Doperation.

SUMMARY OF THE INVENTION

The present invention provides a D2D signal transmitting methodimplemented by a terminal in a wireless communication system and aterminal using the said method.

In one aspect, provided is a method for a device-to-device (D2D)operation performed by a terminal in a radio resource control (RRC) idlestate in a wireless communication system. The method includesdetermining whether a serving cell of the terminal is a suitable cell,determining whether the serving cell provides resource pool informationand transmitting a D2D signal within a resource indicated by theresource pool information when the serving cell is the suitable cell andthe serving cell provides the resource pool information.

The D2D signal may be a D2D discovery signal.

The resource pool information may be provided while being included insystem information.

The resource pool information may indicate a plurality of resourcepools.

A resource pool may be selected among the plurality of resource pools,and the D2D signal may be transmitted by using the selected resourcepool.

In another aspect, provided is a terminal. The terminal includes a radiofrequency (RF) unit transmitting and receiving a radio signal and aprocessor operated in association with the RF unit. The processordetermines whether a serving cell of the terminal is a suitable cell,determines whether the serving cell provides resource pool information,and transmits a D2D signal within a resource indicated by the resourcepool information when the serving cell is the suitable cell and theserving cell provides the resource pool information.

According to the present invention, it is prescribed by which resourceand under which condition a terminal in an RRC idle state can transmit aD2D signal. The terminal is prevented from performing an inappropriateD2D operation by clarifying the D2D operation of the terminal to preventthe terminal from interfering in a network or the terminal from performan operation out of a control of the network by performing an unintendedterminal operation. Therefore, reliability of the D2D operation can beincreased and operation reliability of the network supporting D2D can beimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system to which the presentinvention is applied.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane.

FIG. 3 is a diagram showing a wireless protocol architecture for acontrol plane.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate.

FIG. 5 is a flowchart illustrating a process of establishing RRCconnection.

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocess.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

FIG. 8 illustrates substates which may be owned by UE in the RRC_IDLEstate and a substate transition process.

FIG. 9 shows a basic structure for ProSe.

FIG. 10 shows the deployment examples of types of UE performing ProSedirect communication and cell coverage.

FIG. 11 shows a user plane protocol stack for ProSe directcommunication.

FIG. 12 shows the PC 5 interface for D2D direct discovery.

FIG. 13 is an embodiment of a ProSe discovery process.

FIG. 14 is another embodiment of a ProSe discovery process.

FIG. 15 illustrates a case in which the present invention can beapplied.

FIG. 16 illustrates the D2D operation of the terminal according to thefirst method.

FIG. 17 illustrates a D2D operating method of a terminal according to anembodiment of the present invention.

FIG. 18 illustrates a D2D operating method of a terminal according to anembodiment of the present invention.

FIG. 19 is a block diagram illustrating a terminal in which anembodiment of the present invention is implemented.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a wireless communication system to which the presentinvention is applied. The wireless communication system may also bereferred to as an evolved-UMTS terrestrial radio access network(E-UTRAN) or a long term evolution (LTE)/LTE-A system.

The E-UTRAN includes at least one base station (BS) 20 which provides acontrol plane and a user plane to a user equipment (UE) 10. The UE 10may be fixed or mobile, and may be referred to as another terminology,such as a mobile station (MS), a user terminal (UT), a subscriberstation (SS), a mobile terminal (MT), a wireless device, etc. The BS 20is generally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as an evolved node-B (eNB), abase transceiver system (BTS), an access point, etc.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20are also connected by means of an S1 interface to an evolved packet core(EPC) 30, more specifically, to a mobility management entity (MME)through S1-MME and to a serving gateway (S-GW) through S1-U.

The EPC 30 includes an MME, an S-GW, and a packet data network-gateway(P-GW). The MME has access information of the UE or capabilityinformation of the UE, and such information is generally used formobility management of the UE. The S-GW is a gateway having an E-UTRANas an end point. The P-GW is a gateway having a PDN as an end point.

Layers of a radio interface protocol between the UE and the network canbe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. Among them, a physical (PHY) layer belonging to the first layerprovides an information transfer service by using a physical channel,and a radio resource control (RRC) layer belonging to the third layerserves to control a radio resource between the UE and the network. Forthis, the RRC layer exchanges an RRC message between the UE and the BS.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane. FIG. 3 is a diagram showing a wireless protocol architecture fora control plane. The user plane is a protocol stack for user datatransmission. The control plane is a protocol stack for control signaltransmission.

Referring to FIGS. 2 and 3, a PHY layer provides an upper layer with aninformation transfer service through a physical channel. The PHY layeris connected to a medium access control (MAC) layer which is an upperlayer of the PHY layer through a transport channel. Data is transferredbetween the MAC layer and the PHY layer through the transport channel.The transport channel is classified according to how and with whatcharacteristics data is transferred through a radio interface.

Data is moved between different PHY layers, that is, the PHY layers of atransmitter and a receiver, through a physical channel. The physicalchannel may be modulated according to an Orthogonal Frequency DivisionMultiplexing (OFDM) scheme, and use the time and frequency as radioresources.

The functions of the MAC layer include mapping between a logical channeland a transport channel and multiplexing and demultiplexing to atransport block that is provided through a physical channel on thetransport channel of a MAC Service Data Unit (SDU) that belongs to alogical channel. The MAC layer provides service to a Radio Link Control(RLC) layer through the logical channel.

The functions of the RLC layer include the concatenation, segmentation,and reassembly of an RLC SDU. In order to guarantee various types ofQuality of Service (QoS) required by a Radio Bearer (RB), the RLC layerprovides three types of operation mode: Transparent Mode (TM),Unacknowledged Mode (UM), and Acknowledged Mode (AM). AM RLC provideserror correction through an Automatic Repeat Request (ARQ).

The RRC layer is defined only on the control plane. The RRC layer isrelated to the configuration, reconfiguration, and release of radiobearers, and is responsible for control of logical channels, transportchannels, and PHY channels. An RB means a logical route that is providedby the first layer (PHY layer) and the second layers (MAC layer, the RLClayer, and the PDCP layer) in order to transfer data between UE and anetwork.

The function of a Packet Data Convergence Protocol (PDCP) layer on theuser plane includes the transfer of user data and header compression andciphering. The function of the PDCP layer on the user plane furtherincludes the transfer and encryption/integrity protection of controlplane data.

What an RB is configured means a process of defining the characteristicsof a wireless protocol layer and channels in order to provide specificservice and configuring each detailed parameter and operating method. AnRB can be divided into two types of a Signaling RB (SRB) and a Data RB(DRB). The SRB is used as a passage through which an RRC message istransmitted on the control plane, and the DRB is used as a passagethrough which user data is transmitted on the user plane.

If RRC connection is established between the RRC layer of UE and the RRClayer of an E-UTRAN, the UE is in the RRC connected state. If not, theUE is in the RRC idle state.

A downlink transport channel through which data is transmitted from anetwork to UE includes a broadcast channel (BCH) through which systeminformation is transmitted and a downlink shared channel (SCH) throughwhich user traffic or control messages are transmitted. Traffic or acontrol message for downlink multicast or broadcast service may betransmitted through the downlink SCH, or may be transmitted through anadditional downlink multicast channel (MCH). Meanwhile, an uplinktransport channel through which data is transmitted from UE to a networkincludes a random access channel (RACH) through which an initial controlmessage is transmitted and an uplink shared channel (SCH) through whichuser traffic or control messages are transmitted.

Logical channels that are placed over the transport channel and that aremapped to the transport channel include a broadcast control channel(BCCH), a paging control channel (PCCH), a common control channel(CCCH), a multicast control channel (MCCH), and a multicast trafficchannel (MTCH).

The physical channel includes several OFDM symbols in the time domainand several subcarriers in the frequency domain. One subframe includes aplurality of OFDM symbols in the time domain. An RB is a resourcesallocation unit, and includes a plurality of OFDM symbols and aplurality of subcarriers. Furthermore, each subframe may use specificsubcarriers of specific OFDM symbols (e.g., the first OFDM symbol) ofthe corresponding subframe for a physical downlink control channel(PDCCH), that is, an L1/L2 control channel. A Transmission Time Interval(TTI) is a unit time for subframe transmission.

The RRC state of UE and an RRC connection method are described below.

The RRC state means whether or not the RRC layer of UE is logicallyconnected to the RRC layer of the E-UTRAN. A case where the RRC layer ofUE is logically connected to the RRC layer of the E-UTRAN is referred toas an RRC connected state. A case where the RRC layer of UE is notlogically connected to the RRC layer of the E-UTRAN is referred to as anRRC idle state. The E-UTRAN may check the existence of corresponding UEin the RRC connected state in each cell because the UE has RRCconnection, so the UE may be effectively controlled. In contrast, theE-UTRAN is unable to check UE in the RRC idle state, and a Core Network(CN) manages UE in the RRC idle state in each tracking area, that is,the unit of an area greater than a cell. That is, the existence ornon-existence of UE in the RRC idle state is checked only for each largearea. Accordingly, the UE needs to shift to the RRC connected state inorder to be provided with common mobile communication service, such asvoice or data.

When a user first powers UE, the UE first searches for a proper cell andremains in the RRC idle state in the corresponding cell. The UE in theRRC idle state establishes RRC connection with an E-UTRAN through an RRCconnection procedure when it is necessary to set up the RRC connection,and shifts to the RRC connected state. A case where UE in the RRC idlestate needs to set up RRC connection includes several cases. Forexample, the cases may include a need to send uplink data for a reason,such as a call attempt by a user, and to send a response message as aresponse to a paging message received from an E-UTRAN.

A Non-Access Stratum (NAS) layer placed over the RRC layer performsfunctions, such as session management and mobility management.

In the NAS layer, in order to manage the mobility of UE, two types ofstates: EPS Mobility Management-REGISTERED (EMM-REGISTERED) andEMM-DEREGISTERED are defined. The two states are applied to UE and theMME. UE is initially in the EMM-DEREGISTERED state. In order to access anetwork, the UE performs a process of registering it with thecorresponding network through an initial attach procedure. If the attachprocedure is successfully performed, the UE and the MME become theEMM-REGISTERED state.

In order to manage signaling connection between UE and the EPC, twotypes of states: an EPS Connection Management (ECM)-IDLE state and anECM-CONNECTED state are defined. The two states are applied to UE andthe MME. When the UE in the ECM-IDLE state establishes RRC connectionwith the E-UTRAN, the UE becomes the ECM-CONNECTED state. The MME in theECM-IDLE state becomes the ECM-CONNECTED state when it establishes S1connection with the E-UTRAN. When the UE is in the ECM-IDLE state, theE-UTRAN does not have information about the context of the UE.Accordingly, the UE in the ECM-IDLE state performs procedures related toUE-based mobility, such as cell selection or cell reselection, without aneed to receive a command from a network. In contrast, when the UE is inthe ECM-CONNECTED state, the mobility of the UE is managed in responseto a command from a network. If the location of the UE in the ECM-IDLEstate is different from a location known to the network, the UE informsthe network of its corresponding location through a tracking area updateprocedure.

System information is described below.

System information includes essential information that needs to be knownby UE in order for the UE to access a BS. Accordingly, the UE needs tohave received all pieces of system information before accessing the BS,and needs to always have the up-to-date system information. Furthermore,the BS periodically transmits the system information because the systeminformation is information that needs to be known by all UEs within onecell. The system information is divided into a Master Information Block(MIB) and a plurality of System Information Blocks (SIBs).

The MIB may include the limited number of parameters which are the mostessential and are most frequently transmitted in order to obtain otherinformation from a cell. UE first discovers an MIB after downlinksynchronization. The MIB may include information, such as a downlinkchannel bandwidth, a PHICH configuration, an SFN supportingsynchronization and operating as a timing reference, and an eNBtransmission antenna configuration. The MIB may be broadcasted on a BCH.

SystemInformationBlockType1 (SIB1) of included SIBs is included in a“SystemInformationBlockType1” message and transmitted. Other SIBs otherthan the SIB1 are included in a system information message andtransmitted. The mapping of the SIBs to the system information messagemay be flexibly configured by a scheduling information list parameterincluded in the SIB1. In this case, each SIB is included in a singlesystem information message. Only SIBs having the same schedulingrequired value (e.g. period) may be mapped to the same systeminformation message. Furthermore, SystemInformationBlockType2 (SIB2) isalways mapped to a system information message corresponding to the firstentry within the system information message list of a schedulinginformation list. A plurality of system information messages may betransmitted within the same period. The SIB1 and all of the systeminformation messages are transmitted on a DL-SCH.

In addition to broadcast transmission, in the E-UTRAN, the SIB1 may bechannel-dedicated signaling including a parameter set to have the samevalue as an existing set value. In this case, the SIB1 may be includedin an RRC connection re-establishment message and transmitted.

The SIB1 includes information related to UE cell access and defines thescheduling of other SIBs. The SIB1 may include information related tothe PLMN identifiers, Tracking Area Code (TAC), and cell ID of anetwork, a cell barring state indicative of whether a cell is a cell onwhich UE can camp, a required minimum reception level within a cellwhich is used as a cell reselection reference, and the transmission timeand period of other SIBs.

The SIB2 may include radio resource configuration information common toall types of UE. The SIB2 may include information related to an uplinkcarrier frequency and uplink channel bandwidth, an RACH configuration, apage configuration, an uplink power control configuration, a soundingreference signal configuration, a PUCCH configuration supportingACK/NACK transmission, and a PUSCH configuration.

UE may apply a procedure for obtaining system information and fordetecting a change of system information to only a PCell. In an SCell,when the corresponding SCell is added, the E-UTRAN may provide all typesof system information related to an RRC connection state operationthrough dedicated signaling. When system information related to aconfigured SCell is changed, the E-UTRAN may release a considered SCelland add the considered SCell later. This may be performed along with asingle RRC connection re-establishment message. The E-UTRAN may set avalue broadcast within a considered SCell and other parameter valuethrough dedicated signaling.

UE needs to guarantee the validity of a specific type of systeminformation. Such system information is called required systeminformation. The required system information may be defined as follows.

-   -   If UE is in the RRC_IDLE state: the UE needs to have the valid        version of the MIB and the SIB1 in addition to the SIB2 to SIBS.        This may comply with the support of a considered RAT.    -   If UE is in the RRC connection state: the UE needs to have the        valid version of the MIB, SIB1, and SIB2.

In general, the validity of system information may be guaranteed up to amaximum of 3 hours after being obtained.

In general, service that is provided to UE by a network may beclassified into three types as follows. Furthermore, the UE differentlyrecognizes the type of cell depending on what service may be provided tothe UE. In the following description, a service type is first described,and the type of cell is described.

1) Limited service: this service provides emergency calls and anEarthquake and Tsunami Warning System (ETWS), and may be provided by anacceptable cell.

2) Suitable service: this service means public service for common uses,and may be provided by a suitable cell (or a normal cell).

3) Operator service: this service means service for communicationnetwork operators. This cell may be used by only communication networkoperators, but may not be used by common users.

In relation to a service type provided by a cell, the type of cell maybe classified as follows.

1) An acceptable cell: this cell is a cell from which UE may be providedwith limited service. This cell is a cell that has not been barred froma viewpoint of corresponding UE and that satisfies the cell selectioncriterion of the UE.

2) A suitable cell: this cell is a cell from which UE may be providedwith suitable service. This cell satisfies the conditions of anacceptable cell and also satisfies additional conditions. The additionalconditions include that the suitable cell needs to belong to a PublicLand Mobile Network (PLMN) to which corresponding UE may access and thatthe suitable cell is a cell on which the execution of a tracking areaupdate procedure by the UE is not barred. If a corresponding cell is aCSG cell, the cell needs to be a cell to which UE may access as a memberof the CSG.

3) A barred cell: this cell is a cell that broadcasts informationindicative of a barred cell through system information.

4) A reserved cell: this cell is a cell that broadcasts informationindicative of a reserved cell through system information.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate. FIG. 4 illustrates a procedure in which UE that is initiallypowered on experiences a cell selection process, registers it with anetwork, and then performs cell reselection if necessary.

Referring to FIG. 4, the UE selects Radio Access Technology (RAT) inwhich the UE communicates with a Public Land Mobile Network (PLMN), thatis, a network from which the UE is provided with service (S410).Information about the PLMN and the RAT may be selected by the user ofthe UE, and the information stored in a Universal Subscriber IdentityModule (USIM) may be used.

The UE selects a cell that has the greatest value and that belongs tocells having measured BS and signal intensity or quality greater than aspecific value (cell selection) (S420). In this case, the UE that ispowered off performs cell selection, which may be called initial cellselection. A cell selection procedure is described later in detail.After the cell selection, the UE receives system informationperiodically by the BS. The specific value refers to a value that isdefined in a system in order for the quality of a physical signal indata transmission/reception to be guaranteed. Accordingly, the specificvalue may differ depending on applied RAT.

If network registration is necessary, the UE performs a networkregistration procedure (S430). The UE registers its information (e.g.,an IMSI) with the network in order to receive service (e.g., paging)from the network. The UE does not register it with a network whenever itselects a cell, but registers it with a network when information aboutthe network (e.g., a Tracking Area Identity (TAI)) included in systeminformation is different from information about the network that isknown to the UE.

The UE performs cell reselection based on a service environment providedby the cell or the environment of the UE (S440). If the value of theintensity or quality of a signal measured based on a BS from which theUE is provided with service is lower than that measured based on a BS ofa neighboring cell, the UE selects a cell that belongs to other cellsand that provides better signal characteristics than the cell of the BSthat is accessed by the UE. This process is called cell reselectiondifferently from the initial cell selection of the No. 2 process. Inthis case, temporal restriction conditions are placed in order for acell to be frequently reselected in response to a change of signalcharacteristic. A cell reselection procedure is described later indetail.

FIG. 5 is a flowchart illustrating a process of establishing RRCconnection.

UE sends an RRC connection request message that requests RRC connectionto a network (S510). The network sends an RRC connection establishmentmessage as a response to the RRC connection request (S520). Afterreceiving the RRC connection establishment message, the UE enters RRCconnected mode.

The UE sends an RRC connection establishment complete message used tocheck the successful completion of the RRC connection to the network(S530).

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocess. An RRC connection reconfiguration is used to modify RRCconnection. This is used to establish/modify/release RBs, performhandover, and set up/modify/release measurements.

A network sends an RRC connection reconfiguration message for modifyingRRC connection to UE (S610). As a response to the RRC connectionreconfiguration message, the UE sends an RRC connection reconfigurationcomplete message used to check the successful completion of the RRCconnection reconfiguration to the network (S620).

Hereinafter, a public land mobile network (PLMN) is described.

The PLMN is a network which is disposed and operated by a mobile networkoperator. Each mobile network operator operates one or more PLMNs. EachPLMN may be identified by a Mobile Country Code (MCC) and a MobileNetwork Code (MNC). PLMN information of a cell is included in systeminformation and broadcasted.

In PLMN selection, cell selection, and cell reselection, various typesof PLMNs may be considered by the terminal.

Home PLMN (HPLMN): PLMN having MCC and MNC matching with MCC and MNC ofa terminal IMSI.

Equivalent HPLMN (EHPLMN): PLMN serving as an equivalent of an HPLMN.

Registered PLMN (RPLMN): PLMN successfully finishing locationregistration.

Equivalent PLMN (EPLMN): PLMN serving as an equivalent of an RPLMN.

Each mobile service consumer subscribes in the HPLMN. When a generalservice is provided to the terminal through the HPLMN or the EHPLMN, theterminal is not in a roaming state. Meanwhile, when the service isprovided to the terminal through a PLMN except for the HPLMN/EHPLMN, theterminal is in the roaming state. In this case, the PLMN refers to aVisited PLMN (VPLMN).

When UE is initially powered on, the UE searches for available PublicLand Mobile Networks (PLMNs) and selects a proper PLMN from which the UEis able to be provided with service. The PLMN is a network that isdeployed or operated by a mobile network operator. Each mobile networkoperator operates one or more PLMNs. Each PLMN may be identified byMobile Country Code (MCC) and Mobile Network Code (MNC). Informationabout the PLMN of a cell is included in system information andbroadcasted. The UE attempts to register it with the selected PLMN. Ifregistration is successful, the selected PLMN becomes a Registered PLMN(RPLMN). The network may signalize a PLMN list to the UE. In this case,PLMNs included in the PLMN list may be considered to be PLMNs, such asRPLMNs. The UE registered with the network needs to be able to be alwaysreachable by the network. If the UE is in the ECM-CONNECTED state(identically the RRC connection state), the network recognizes that theUE is being provided with service. If the UE is in the ECM-IDLE state(identically the RRC idle state), however, the situation of the UE isnot valid in an eNB, but is stored in the MME. In such a case, only theMME is informed of the location of the UE in the ECM-IDLE state throughthe granularity of the list of Tracking Areas (TAs). A single TA isidentified by a Tracking Area Identity (TAI) formed of the identifier ofa PLMN to which the TA belongs and Tracking Area Code (TAC) thatuniquely expresses the TA within the PLMN.

Thereafter, the UE selects a cell that belongs to cells provided by theselected PLMN and that has signal quality and characteristics on whichthe UE is able to be provided with proper service.

The following is a detailed description of a procedure of selecting acell by a terminal.

When power is turned-on or the terminal is located in a cell, theterminal performs procedures for receiving a service byselecting/reselecting a suitable quality cell.

A terminal in an RRC idle state should prepare to receive a servicethrough the cell by always selecting a suitable quality cell. Forexample, a terminal where power is turned-on just before should selectthe suitable quality cell to be registered in a network. If the terminalin an RRC connection state enters in an RRC idle state, the terminalshould selects a cell for stay in the RRC idle state. In this way, aprocedure of selecting a cell satisfying a certain condition by theterminal in order to be in a service idle state such as the RRC idlestate refers to cell selection. Since the cell selection is performed ina state that a cell in the RRC idle state is not currently determined,it is important to select the cell as rapid as possible. Accordingly, ifthe cell provides a wireless signal quality of a predetermined level orgreater, although the cell does not provide the best wireless signalquality, the cell may be selected during a cell selection procedure ofthe terminal.

A method and a procedure of selecting a cell by a terminal in a 3GPP LTEis described with reference to 3GPP TS 36.304 V8.5.0 (2009-03) “UserEquipment (UE) procedures in idle mode (Release 8)”.

A cell selection process is basically divided into two types.

The first is an initial cell selection process. In this process, UE doesnot have preliminary information about a wireless channel. Accordingly,the UE searches for all wireless channels in order to find out a propercell. The UE searches for the strongest cell in each channel.Thereafter, if the UE has only to search for a suitable cell thatsatisfies a cell selection criterion, the UE selects the correspondingcell.

Next, the UE may select the cell using stored information or usinginformation broadcasted by the cell. Accordingly, cell selection may befast compared to an initial cell selection process. If the UE has onlyto search for a cell that satisfies the cell selection criterion, the UEselects the corresponding cell. If a suitable cell that satisfies thecell selection criterion is not retrieved though such a process, the UEperforms an initial cell selection process.

The cell selection criterion may be defined as below equation 1.Srxlev>0 AND Squal>0where:Srxlev=Q _(rxlevmeas) −Q _(rxlevmin) +Q _(rxlevminoffset))−PcompensationSqual=Q _(qualmeas)−(Q _(qualmin) +Q _(qualminoffset))  [Equation 1]

Here, the variables in the equation 1 may be defined as below table 1.

TABLE 1 Srxlev Cell selection RX level value (dB) Squal Cell selectionquality value (dB) Q_(rxlevmeas) Measured cell RX level value (RSRP)Q_(qualmeas) Measured cell quality value (RSRQ) Q_(rxlevmin) Minimumrequired RX level in the cell (dBm) Q_(qualmin) Minimum required qualitylevel in the cell (dB) Q_(rxlevminoffset) Offset to the signalledQ_(rxlevmin) taken into account in the Srxlev evaluation as a result ofa periodic search for a higher priority PLMN while camped normally in aVPLMN Q_(qualminoffset) Offset to the signalled Q_(qualmin) taken intoaccount in the Squal evaluation as a result of a periodic search for ahigher priority PLMN while camped normally in a VPLMN Pcompensationmax(P_(EMAX) − P_(PowerClass,) 0) (dB) P_(EMAX) Maximum TX power levelan UE may use when transmitting on the uplink in the cell (dBm) definedas P_(EMAX) in [TS 36.101] P_(PowerClass) Maximum RF output power of theUE (dBm) according to the UE power class as defined in [TS 36.101]

Signalled values, i.e., Q_(rxlevminoffset) and Q_(qualminoffset), may beapplied to a case where cell selection is evaluated as a result ofperiodic search for a higher priority PLMN during a UE camps on a normalcell in a VPLMN. During the periodic search for the higher priority PLMNas described above, the UE may perform the cell selection evaluation byusing parameter values stored in other cells of the higher priorityPLMN.

After the UE selects a specific cell through the cell selection process,the intensity or quality of a signal between the UE and a BS may bechanged due to a change in the mobility or wireless environment of theUE. Accordingly, if the quality of the selected cell is deteriorated,the UE may select another cell that provides better quality. If a cellis reselected as described above, the UE selects a cell that providesbetter signal quality than the currently selected cell. Such a processis called cell reselection. In general, a basic object of the cellreselection process is to select a cell that provides UE with the bestquality from a viewpoint of the quality of a radio signal.

In addition to the viewpoint of the quality of a radio signal, a networkmay determine priority corresponding to each frequency, and may informthe UE of the determined priorities. The UE that has received thepriorities preferentially takes into consideration the priorities in acell reselection process compared to a radio signal quality criterion.

As described above, there is a method of selecting or reselecting a cellaccording to the signal characteristics of a wireless environment. Inselecting a cell for reselection when a cell is reselected, thefollowing cell reselection methods may be present according to the RATand frequency characteristics of the cell.

-   -   Intra-frequency cell reselection: UE reselects a cell having the        same center frequency as that of RAT, such as a cell on which        the UE camps on.    -   Inter-frequency cell reselection: UE reselects a cell having a        different center frequency from that of RAT, such as a cell on        which the UE camps on    -   Inter-RAT cell reselection: UE reselects a cell that uses RAT        different from RAT on which the UE camps

The principle of a cell reselection process is as follows.

First, UE measures the quality of a serving cell and neighbor cells forcell reselection.

Second, cell reselection is performed based on a cell reselectioncriterion. The cell reselection criterion has the followingcharacteristics in relation to the measurements of a serving cell andneighbor cells.

Intra-frequency cell reselection is basically based on ranking. Rankingis a task for defining a criterion value for evaluating cell reselectionand numbering cells using criterion values according to the size of thecriterion values. A cell having the best criterion is commonly calledthe best-ranked cell. The cell criterion value is based on the value ofa corresponding cell measured by UE, and may be a value to which afrequency offset or cell offset has been applied, if necessary.

Inter-frequency cell reselection is based on frequency priority providedby a network. UE attempts to camp on a frequency having the highestfrequency priority. A network may provide frequency priority that willbe applied by UEs within a cell in common through broadcastingsignaling, or may provide frequency-specific priority to each UE throughUE-dedicated signaling. A cell reselection priority provided throughbroadcast signaling may refer to a common priority. A cell reselectionpriority for each terminal set by a network may refer to a dedicatedpriority. If receiving the dedicated priority, the terminal may receivea valid time associated with the dedicated priority together. Ifreceiving the dedicated priority, the terminal starts a validity timerset as the received valid time together therewith. While the valid timeris operated, the terminal applies the dedicated priority in the RRC idlemode. If the valid timer is expired, the terminal discards the dedicatedpriority and again applies the common priority.

For the inter-frequency cell reselection, a network may provide UE witha parameter (e.g., a frequency-specific offset) used in cell reselectionfor each frequency.

For the intra-frequency cell reselection or the inter-frequency cellreselection, a network may provide UE with a Neighboring Cell List (NCL)used in cell reselection. The NCL includes a cell-specific parameter(e.g., a cell-specific offset) used in cell reselection.

For the intra-frequency or inter-frequency cell reselection, a networkmay provide UE with a cell reselection black list used in cellreselection. The UE does not perform cell reselection on a cell includedin the black list.

Ranking performed in a cell reselection evaluation process is describedbelow.

A ranking criterion used to apply priority to a cell is defined as inEquation 1.Rs=Qmeas,s+Qhyst,Rn=Qmeas,s−Qoffset  [Equation 2]

In this case, Rs is the ranking criterion of a serving cell, Rn is theranking criterion of a neighbor cell, Qmeas,s is the quality value ofthe serving cell measured by UE, Qmeas,n is the quality value of theneighbor cell measured by UE, Qhyst is the hysteresis value for ranking,and Qoffset is an offset between the two cells.

In Intra-frequency, if UE receives an offset “Qoffsets,n” between aserving cell and a neighbor cell, Qoffset=Qoffsets,n. If UE does notQoffsets,n, Qoffset=0.

In Inter-frequency, if UE receives an offset “Qoffsets,n” for acorresponding cell, Qoffset=Qoffsets,n+Qfrequency. If UE does notreceive “Qoffsets,n”, Qoffset=Qfrequency.

If the ranking criterion Rs of a serving cell and the ranking criterionRn of a neighbor cell are changed in a similar state, ranking priorityis frequency changed as a result of the change, and UE may alternatelyreselect the twos. Qhyst is a parameter that gives hysteresis to cellreselection so that UE is prevented from to alternately reselecting twocells.

UE measures RS of a serving cell and Rn of a neighbor cell according tothe above equation, considers a cell having the greatest rankingcriterion value to be the best-ranked cell, and reselects the cell.

In accordance with the criterion, it may be checked that the quality ofa cell is the most important criterion in cell reselection. If areselected cell is not a suitable cell, UE excludes a correspondingfrequency or a corresponding cell from the subject of cell reselection.

A Radio Link Failure (RLF) is described below.

UE continues to perform measurements in order to maintain the quality ofa radio link with a serving cell from which the UE receives service. TheUE determines whether or not communication is impossible in a currentsituation due to the deterioration of the quality of the radio link withthe serving cell. If communication is almost impossible because thequality of the serving cell is too low, the UE determines the currentsituation to be an RLF.

If the RLF is determined, the UE abandons maintaining communication withthe current serving cell, selects a new cell through cell selection (orcell reselection) procedure, and attempts RRC connectionre-establishment with the new cell.

In the specification of 3GPP LTE, the following examples are taken ascases where normal communication is impossible.

-   -   A case where UE determines that there is a serious problem in        the quality of a downlink communication link (a case where the        quality of a PCell is determined to be low while performing RLM)        based on the radio quality measured results of the PHY layer of        the UE    -   A case where uplink transmission is problematic because a random        access procedure continues to fail in the MAC sublayer.    -   A case where uplink transmission is problematic because uplink        data transmission continues to fail in the RLC sublayer.    -   A case where handover is determined to have failed.    -   A case where a message received by UE does not pass through an        integrity check.

An RRC connection re-establishment procedure is described in more detailbelow.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

Referring to FIG. 7, UE stops using all the radio bearers that have beenconfigured other than a Signaling Radio Bearer (SRB) #0, and initializesa variety of kinds of sublayers of an Access Stratum (AS) (S710).Furthermore, the UE configures each sublayer and the PHY layer as adefault configuration. In this process, the UE maintains the RRCconnection state.

The UE performs a cell selection procedure for performing an RRCconnection reconfiguration procedure (S720). The cell selectionprocedure of the RRC connection re-establishment procedure may beperformed in the same manner as the cell selection procedure that isperformed by the UE in the RRC idle state, although the UE maintains theRRC connection state.

After performing the cell selection procedure, the UE determines whetheror not a corresponding cell is a suitable cell by checking the systeminformation of the corresponding cell (S730). If the selected cell isdetermined to be a suitable E-UTRAN cell, the UE sends an RRC connectionre-establishment request message to the corresponding cell (S740).

Meanwhile, if the selected cell is determined to be a cell that uses RATdifferent from that of the E-UTRAN through the cell selection procedurefor performing the RRC connection re-establishment procedure, the UEstops the RRC connection re-establishment procedure and enters the RRCidle state (S750).

The UE may be implemented to finish checking whether the selected cellis a suitable cell through the cell selection procedure and thereception of the system information of the selected cell. To this end,the UE may drive a timer when the RRC connection re-establishmentprocedure is started. The timer may be stopped if it is determined thatthe UE has selected a suitable cell. If the timer expires, the UE mayconsider that the RRC connection re-establishment procedure has failed,and may enter the RRC idle state. Such a timer is hereinafter called anRLF timer. In LTE spec TS 36.331, a timer named “T311” may be used as anRLF timer. The UE may obtain the set value of the timer from the systeminformation of the serving cell.

If an RRC connection re-establishment request message is received fromthe UE and the request is accepted, a cell sends an RRC connectionre-establishment message to the UE.

The UE that has received the RRC connection re-establishment messagefrom the cell reconfigures a PDCP sublayer and an RLC sublayer with anSRB1. Furthermore, the UE calculates various key values related tosecurity setting, and reconfigures a PDCP sublayer responsible forsecurity as the newly calculated security key values. Accordingly, theSRB 1 between the UE and the cell is open, and the UE and the cell mayexchange RRC control messages. The UE completes the restart of the SRB1,and sends an RRC connection re-establishment complete message indicativeof that the RRC connection re-establishment procedure has been completedto the cell (S760).

In contrast, if the RRC connection re-establishment request message isreceived from the UE and the request is not accepted, the cell sends anRRC connection re-establishment reject message to the UE.

If the RRC connection re-establishment procedure is successfullyperformed, the cell and the UE perform an RRC connection reconfigurationprocedure. Accordingly, the UE recovers the state prior to the executionof the RRC connection re-establishment procedure, and the continuity ofservice is guaranteed to the upmost.

FIG. 8 illustrates substates which may be owned by UE in the RRC_IDLEstate and a substate transition process.

Referring to FIG. 8, UE performs an initial cell selection process(S801). The initial cell selection process may be performed when thereis no cell information stored with respect to a PLMN or if a suitablecell is not discovered.

If a suitable cell is unable to be discovered in the initial cellselection process, the UE transits to any cell selection state (S802).The any cell selection state is the state in which the UE has not campedon a suitable cell and an acceptable cell and is the state in which theUE attempts to discover an acceptable cell of a specific PLMN on whichthe UE may camp. If the UE has not discovered any cell on which it maycamp, the UE continues to stay in the any cell selection state until itdiscovers an acceptable cell.

If a suitable cell is discovered in the initial cell selection process,the UE transits to a normal camp state (S803). The normal camp staterefers to the state in which the UE has camped on the suitable cell. Inthis state, the UE may select and monitor a paging channel based oninformation provided through system information and may perform anevaluation process for cell reselection.

If a cell reselection evaluation process (S804) is caused in the normalcamp state (S803), the UE performs a cell reselection evaluation process(S804). If a suitable cell is discovered in the cell reselectionevaluation process (S804), the UE transits to the normal camp state(S803) again.

If an acceptable cell is discovered in the any cell selection state(S802), the UE transmits to any cell camp state (S805). The any cellcamp state is the state in which the UE has camped on the acceptablecell.

In the any cell camp state (S805), the UE may select and monitor apaging channel based on information provided through system informationand may perform the evaluation process (S806) for cell reselection. Ifan acceptable cell is not discovered in the evaluation process (S806)for cell reselection, the UE transits to the any cell selection state(S802).

Now, a device-to-device (D2D) operation is described. In 3GPP LTE-A, aservice related to the D2D operation is called a proximity service(ProSe). Now, the ProSe is described. Hereinafter, the ProSe is the sameconcept as the D2D operation, and the ProSe and the D2D operation may beused without distinction.

The ProSe includes ProSe direction communication and ProSe directdiscovery. The ProSe direct communication is communication performedbetween two or more proximate UEs. The UEs may perform communication byusing a protocol of a user plane. A ProSe-enabled UE implies a UEsupporting a procedure related to a requirement of the ProSe. Unlessotherwise specified, the ProSe-enabled UE includes both of a publicsafety UE and a non-public safety UE. The public safety UE is a UEsupporting both of a function specified for a public safety and a ProSeprocedure, and the non-public safety UE is a UE supporting the ProSeprocedure and not supporting the function specified for the publicsafety.

ProSe direct discovery is a process for discovering anotherProSe-enabled UE adjacent to ProSe-enabled UE. In this case, only thecapabilities of the two types of ProSe-enabled UE are used. EPC-levelProSe discovery means a process for determining, by an EPC, whether thetwo types of ProSe-enabled UE are in proximity and notifying the twotypes of ProSe-enabled UE of the proximity.

Hereinafter, for convenience, the ProSe direct communication may bereferred to as D2D communication, and the ProSe direct discovery may bereferred to as D2D discovery.

FIG. 9 shows a basic structure for ProSe.

Referring to FIG. 9, the basic structure for ProSe includes an E-UTRAN,an EPC, a plurality of types of UE including a ProSe applicationprogram, a ProSe application server (a ProSe APP server), and a ProSefunction.

The EPC represents an E-UTRAN core network configuration. The EPC mayinclude an MME, an S-GW, a P-GW, a policy and charging rules function(PCRF), a home subscriber server (HSS) and so on.

The ProSe APP server is a user of a ProSe capability for producing anapplication function. The ProSe APP server may communicate with anapplication program within UE. The application program within UE may usea ProSe capability for producing an application function.

The ProSe function may include at least one of the followings, but isnot necessarily limited thereto.

-   -   Interworking via a reference point toward the 3rd party        applications    -   Authorization and configuration of UE for discovery and direct        communication    -   Enable the functionality of EPC level ProSe discovery    -   ProSe related new subscriber data and handling of data storage,        and also handling of the ProSe identities    -   Security related functionality    -   Provide control towards the EPC for policy related functionality    -   Provide functionality for charging (via or outside of the EPC,        e.g., offline charging)

A reference point and a reference interface in the basic structure forProSe are described below.

-   -   PC1: a reference point between the ProSe application program        within the UE and the ProSe application program within the ProSe        APP server. This is used to define signaling requirements in an        application dimension.    -   PC2: a reference point between the ProSe APP server and the        ProSe function. This is used to define an interaction between        the ProSe APP server and the ProSe function. The update of        application data in the ProSe database of the ProSe function may        be an example of the interaction.    -   PC3: a reference point between the UE and the ProSe function.        This is used to define an interaction between the UE and the        ProSe function. A configuration for ProSe discovery and        communication may be an example of the interaction.    -   PC4: a reference point between the EPC and the ProSe function.        This is used to define an interaction between the EPC and the        ProSe function. The interaction may illustrate the time when a        path for 1:1 communication between types of UE is set up or the        time when ProSe service for real-time session management or        mobility management is authenticated.    -   PC5: a reference point used for using control/user plane for        discovery and communication, relay, and 1:1 communication        between types of UE.    -   PC6: a reference point for using a function, such as ProSe        discovery, between users belonging to different PLMNs.    -   SGi: this may be used to exchange application data and types of        application dimension control information.

<ProSe Direct Communication>

ProSe direct communication is communication mode in which two types ofpublic safety UE can perform direct communication through a PC 5interface. Such communication mode may be supported when UE is suppliedwith services within coverage of an E-UTRAN or when UE deviates fromcoverage of an E-UTRAN.

FIG. 10 shows the deployment examples of types of UE performing ProSedirect communication and cell coverage.

Referring to FIG. 10(a), types of UE A and B may be placed outside cellcoverage. Referring to FIG. 10(b), UE A may be placed within cellcoverage, and UE B may be placed outside cell coverage. Referring toFIG. 10(c), types of UE A and B may be placed within single cellcoverage. Referring to FIG. 10(d), UE A may be placed within coverage ofa first cell, and UE B may be placed within coverage of a second cell.

ProSe direct communication may be performed between types of UE placedat various positions as in FIG. 10.

Meanwhile, the following IDs may be used in ProSe direct communication.

A source layer-2 ID: this ID identifies the sender of a packet in the PC5 interface.

A destination layer-2 ID: this ID identifies the target of a packet inthe PC 5 interface.

An SA L1 ID: this ID is the ID of scheduling assignment (SA) in the PC 5interface.

FIG. 11 shows a user plane protocol stack for ProSe directcommunication.

Referring to FIG. 11, the PC 5 interface includes a PDCH, RLC, MAC, andPHY layers.

In ProSe direct communication, HARQ feedback may not be present. An MACheader may include a source layer-2 ID and a destination layer-2 ID.

<Radio Resource Assignment for ProSe Direct Communication>

ProSe-enabled UE may use the following two types of mode for resourceassignment for ProSe direct communication.

1. Mode 1

Mode 1 is mode in which resources for ProSe direct communication arescheduled by an eNB. UE needs to be in the RRC_CONNECTED state in orderto send data in accordance with mode 1. The UE requests a transmissionresource from an eNB. The eNB performs scheduling assignment andschedules resources for sending data. The UE may send a schedulingrequest to the eNB and send a ProSe Buffer Status Report (BSR). The eNBhas data to be subjected to ProSe direct communication by the UE basedon the ProSe BSR and determines that a resource for transmission isrequired.

2. Mode 2

Mode 2 is mode in which UE directly selects a resource. UE directlyselects a resource for ProSe direct communication in a resource pool.The resource pool may be configured by a network or may have beenpreviously determined.

Meanwhile, if UE has a serving cell, that is, if the UE is in theRRC_CONNECTED state with an eNB or is placed in a specific cell in theRRC_IDLE state, the UE is considered to be placed within coverage of theeNB.

If UE is placed outside coverage, only mode 2 may be applied. If the UEis placed within the coverage, the UE may use mode 1 or mode 2 dependingon the configuration of an eNB.

If another exception condition is not present, only when an eNB performsa configuration, UE may change mode from mode 1 to mode 2 or from mode 2to mode 1.

<ProSe Direct Discovery>

ProSe direct discovery refers to a procedure that is used forProSe-enabled UE to discover another ProSe-enabled UE in proximity andis also called D2D direct discovery. In this case, E-UTRA radio signalsthrough the PC 5 interface may be used. Information used in ProSe directdiscovery is hereinafter called discovery information.

FIG. 12 shows the PC 5 interface for D2D direct discovery.

Referring to FIG. 12, the PC 5 interface includes an MAC layer, a PHYlayer, and a ProSe Protocol layer, that is, a higher layer. The higherlayer (the ProSe Protocol) handles the permission of the announcementand monitoring of discovery information. The contents of the discoveryinformation are transparent to an access stratum (AS). The ProSeProtocol transfers only valid discovery information to the AS forannouncement.

The MAC layer receives discovery information from the higher layer (theProSe Protocol). An IP layer is not used to send discovery information.The MAC layer determines a resource used to announce discoveryinformation received from the higher layer. The MAC layer produces anMAC protocol data unit (PDU) for carrying discovery information andsends the MAC PDU to the physical layer. An MAC header is not added.

In order to announce discovery information, there are two types ofresource assignment.

1. Type 1

As a method in which resources for announcement of discoveredinformation are allocated not specifically to a terminal, a base stationprovides a resource pool configuration for announcement of thediscovered information to terminals. The configuration is included in asystem information block (SIB) to be signaled by a broadcast scheme.Alternatively, the configuration may be provided while being included ina terminal specific RRC message. Alternatively, the configuration may bebroadcast signaling of another layer except for an RRC message orterminal specific signaling.

The terminal autonomously selects the resource from an indicatedresource pool and announces the discovery information by using theselected resource. The terminal may announce the discovery informationthrough an arbitrarily selected resource during each discovery period.

2. Type 2

The type 2 is a method for assigning a resource for announcing discoveryinformation in a UE-specific manner. UE in the RRC_CONNECTED state mayrequest a resource for discovery signal announcement from an eNB throughan RRC signal. The eNB may announce a resource for discovery signalannouncement through an RRC signal. A resource for discovery signalmonitoring may be assigned within a resource pool configured for typesof UE.

An eNB 1) may announce a type 1 resource pool for discovery signalannouncement to UE in the RRC_IDLE state through the SIB. Types of UEwhose ProSe direct discovery has been permitted use the type 1 resourcepool for discovery information announcement in the RRC_IDLE state.Alternatively, the eNB 2) announces that the eNB supports ProSe directdiscovery through the SIB, but may not provide a resource for discoveryinformation announcement. In this case, UE needs to enter theRRC_CONNECTED state for discovery information announcement.

An eNB may configure that UE has to use a type 1 resource pool fordiscovery information announcement or has to use a type 2 resourcethrough an RRC signal in relation to UE in the RRC_CONNECTED state.

FIG. 13 is an embodiment of a ProSe discovery process.

Referring to FIG. 13, it is assumed that UE A and UE B haveProSe-enabled application programs managed therein and have beenconfigured to have a ‘friend’ relation between them in the applicationprograms, that is, a relationship in which D2D communication may bepermitted between them. Hereinafter, the UE B may be represented as a‘friend’ of the UE A. The application program may be, for example, asocial networking program. ‘3GPP Layers’ correspond to the functions ofan application program for using ProSe discovery service, which havebeen defined by 3GPP.

Direct discovery between the types of UE A and B may experience thefollowing process.

1. First, the UE A performs regular application layer communication withthe APP server. The communication is based on an Application ProgramInterface (API).

2. The ProSe-enabled application program of the UE A receives a list ofapplication layer IDs having a ‘friend’ relation. In general, theapplication layer ID may have a network access ID form. For example, theapplication layer ID of the UE A may have a form, such as“adam@example.com.”

3. The UE A requests private expressions code for the user of the UE Aand private representation code for a friend of the user.

4. The 3GPP layers send a representation code request to the ProSeserver.

5. The ProSe server maps the application layer IDs, provided by anoperator or a third party APP server, to the private representationcode. For example, an application layer ID, such as adam@example.com,may be mapped to private representation code, such as“GTER543$#2FSJ67DFSF.” Such mapping may be performed based on parameters(e.g., a mapping algorithm, a key value and so on) received from the APPserver of a network.

6. The ProSe server sends the types of derived representation code tothe 3GPP layers. The 3GPP layers announce the successful reception ofthe types of representation code for the requested application layer IDto the ProSe-enabled application program. Furthermore, the 3GPP layersgenerate a mapping table between the application layer ID and the typesof representation code.

7. The ProSe-enabled application program requests the 3GPP layers tostart a discovery procedure. That is, the ProSe-enabled applicationprogram requests the 3GPP layers to start discovery when one of provided‘friends’ is placed in proximity to the UE A and direct communication ispossible. The 3GPP layers announces the private representation code(i.e., in the above example, “GTER543$#2FSJ67DFSF”, that is, the privaterepresentation code of adam@example.com) of the UE A. This ishereinafter called ‘announcement’. Mapping between the application layerID of the corresponding application program and the privaterepresentation code may be known to only ‘friends’ which have previouslyreceived such a mapping relation, and the ‘friends’ may perform suchmapping.

8. It is assumed that the UE B operates the same ProSe-enabledapplication program as the UE A and has executed the aforementioned 3 to6 steps. The 3GPP layers placed in the UE B may execute ProSe discovery.

9. When the UE B receives the aforementioned ‘announce’ from the UE A,the UE B determines whether the private representation code included inthe ‘announce’ is known to the UE B and whether the privaterepresentation code is mapped to the application layer ID. As describedthe 8 step, since the UE B has also executed the 3 to 6 steps, it isaware of the private representation code, mapping between the privaterepresentation code and the application layer ID, and correspondingapplication program of the UE A. Accordingly, the UE B may discover theUE A from the ‘announce’ of the UE A. The 3GPP layers announce thatadam@example.com has been discovered to the ProSe-enabled applicationprogram within the UE B.

In FIG. 13, the discovery procedure has been described by taking intoconsideration all of the types of UE A and B, the ProSe server, the APPserver and so on. From the viewpoint of the operation between the typesof UE A and B, the UE A sends (this process may be called announcement)a signal called announcement, and the UE B receives the announce anddiscovers the UE A. That is, from the aspect that an operation thatbelongs to operations performed by types of UE and that is directlyrelated to another UE is only step, the discovery process of FIG. 13 mayalso be called a single step discovery procedure.

FIG. 14 is another embodiment of a ProSe discovery process.

In FIG. 14, types of UE 1 to 4 are assumed to types of UE included inspecific group communication system enablers (GCSE) group. It is assumedthat the UE 1 is a discoverer and the types of UE 2, 3, and 4 arediscoveree. UE 5 is UE not related to the discovery process.

The UE 1 and the UE 2-4 may perform a next operation in the discoveryprocess.

First, the UE 1 broadcasts a target discovery request message (may behereinafter abbreviated as a discovery request message or M1) in orderto discover whether specific UE included in the GCSE group is inproximity. The target discovery request message may include the uniqueapplication program group ID or layer-2 group ID of the specific GCSEgroup. Furthermore, the target discovery request message may include theunique ID, that is, application program private ID of the UE 1. Thetarget discovery request message may be received by the types of UE 2,3, 4, and 5.

The UE 5 sends no response message. In contrast, the types of UE 2, 3,and 4 included in the GCSE group send a target discovery responsemessage (may be hereinafter abbreviated as a discovery response messageor M2) as a response to the target discovery request message. The targetdiscovery response message may include the unique application programprivate ID of UE sending the message.

An operation between types of UE in the ProSe discovery processdescribed with reference to FIG. 14 is described below. The discoverer(the UE 1) sends a target discovery request message and receives atarget discovery response message, that is, a response to the targetdiscovery request message. Furthermore, when the discoveree (e.g., theUE 2) receives the target discovery request message, it sends a targetdiscovery response message, that is, a response to the target discoveryrequest message. Accordingly, each of the types of UE performs theoperation of the 2 step. In this aspect, the ProSe discovery process ofFIG. 14 may be called a 2-step discovery procedure.

In addition to the discovery procedure described in FIG. 14, if the UE 1(the discoverer) sends a discovery conform message (may be hereinafterabbreviated as an M3), that is, a response to the target discoveryresponse message, this may be called a 3-step discovery procedure.

Hereinafter, the present invention will be described.

When a terminal is positioned outside network coverage (cell coverage),the terminal may perform a D2D operation by using a preconfiguredresource. That is, when it is determined that the terminal itself ispositioned outside the network coverage, the terminal may perform D2Doperations such as D2D communication with another terminal and D2Ddiscovery by using a preconfigured resource pool. On the contrary, whenthe terminal is positioned within the network coverage, it is aprinciple to perform the D2D operation by using a resource poolcontrolled by a network and in particular, since transmission of a D2Dsignal may interfere in another terminal, the D2D operation needs to beperformed under the control of the network. That is, when the terminalis positioned within the network coverage, the terminal may need toperform the D2D operation by using the resource pool signaled by thenetwork.

In the D2D operation, when or under which condition the terminal needsto be controlled by the network needs to be definitely prescribed. Forexample, in the D2D operation, when the resource pool signaled by thenetwork instead of the preconfigured resource used outside the networkcoverage needs to start to be applied needs to be definitely prescribed.

FIG. 15 illustrates a case in which the present invention can beapplied.

Referring to FIG. 15, the network coverage may be diversely divided intofirst coverage 151, second coverage 152, and third coverage 153. Thefirst coverage 151 is coverage in which stable connection between theterminal and the network is available. The second coverage 152 may becoverage in which the terminal may receive a synchronization signal andsystem information, but transmission power is short for the terminal totransmit an uplink signal to the network. The third coverage 153 may becoverage in which the terminal may detect only the synchronizationsignal. A terminal positioned outside the third coverage 153 may detectno signal from the network.

The terminal may perform the D2D operation by using the preconfiguredresource outside the third coverage 153.

A case in which the terminal is positioned inside the third coverage 153and positioned outside the second coverage 152 is assumed. In this case,the terminal may receive the synchronization signal from the network,but may not receive the system information. Therefore, since it isdifficult that the terminal is substantially positioned within thenetwork coverage and the terminal may move out of the third coverage 153again, changing/switching the resource pool applied to the D2D operationwith respect to the terminal may not be preferable. Only when theterminal moves into the second coverage 152 or the first coverage 151,changing/switching the resource pool applied to the D2D operation may berequired.

Meanwhile, when the terminal camps on a suitable cell and otherwise, aservice which may be provided to the terminal may preferably vary.

As such, the resource pool signaled by the network needs to becontrolled by more precisely dividing an area/a condition/a state, andthe like which the terminal needs to apply. To this end, the presentinvention proposes when (alternatively, under which condition) thenetwork will perform the D2D operation by using resource pool signaledby the network.

As a first method, when the terminal camps on the suitable cell, in thecase where the suitable cell continuously signals information indicatingthe resource pool, the terminal may perform the D2D operation by usingthe signaled resource pool. If the terminal in the RRC idle state is inan any cell selection state, the terminal stops using resource poolinformation which a serving cell has signaled most recently.

If the terminal in the RRC idle state is in the any cell selection stateor a camped on any cell state, the terminal stops using the resourcepool information which the serving cell has signaled most recently.

That is, if the terminal is in the RRC idle state, the terminal uses theresource pool information signaled by the serving cell only in thecamped normally state.

The resource pool information used only when the terminal is in thecamped normally state may indicate a resource for D2D transmission.

In the case where the D2D operation is D2D direct communication, whenthe terminal in the RRC idle state, which is positioned within thenetwork coverage intends to perform transmission of the D2D directcommunication, it is determined that the terminal may perform the D2Dtransmission only when the serving cell of the terminal is the suitablecell. Therefore, when the terminal is in a camped on acceptable cellstate, that is, in the camped on any cell state or in the any cellselection state, the terminal determines not to stop or start the D2Dtransmission.

In the case where the D2D operation is D2D direct discovery, when theterminal in the RRC idle state, which is positioned within the networkcoverage intends to perform D2D discovery announcement, the terminal mayannounce a D2D discovery message only when the serving cell of theterminal is the suitable cell and the terminal may not stop or startannouncement of the D2D discovery message when the serving cell of theterminal is not the suitable cell or the terminal is in the any cellselection state.

The resource pool information used only when the terminal is in thecamped normally state may indicate a resource for D2D reception. In thecase where the D2D operation is the D2D direct communication, when theterminal in the RRC idle state, which is positioned within the networkcoverage intends to perform reception of the D2D direct communication,it is determined that the terminal may perform the D2D reception onlywhen the serving cell of the terminal is the suitable cell. Therefore,when the terminal is in the camped on acceptable cell state, that is, inthe camped on any cell state or in the any cell selection state, theterminal determines to stop or not to start the D2D reception.Meanwhile, in the case where the D2D operation is the D2D directdiscovery, when the terminal in the RRC idle state, which is positionedwithin the network coverage intends to perform D2D discovery monitoring,restriction in performing the D2D discovery monitoring depending on thestate of the terminal based on a serving cell camping situation of theterminal may not be required when it is considered that a D2D discoveryservice is a kind of best-effort service which may be performed within arange not to influence cellular communication. That is, exceptionally,when the terminal performs a D2D discovery monitoring operation, theterminal may perform the D2D discovery monitoring regardless of whetherthe terminal is in the camped normally state, in the any cell campstate, or in the any cell selection state.

When the terminal performs the D2D operation at a serving frequency, theterminal preferably determines whether to perform the D2D operationaccording to whether the terminal camps on the serving cell and whetherthe terminal camps on any serving cell of the suitable cell and anycell.

Unlike this, when a frequency at which the terminal intends to performthe D2D operation and the serving frequency are different from eachother, it may not be inappropriate to determine whether to perform theD2D operation according to the state of the terminal based on theserving cell camping situation of the terminal. In this case, theterminal may select a cell for the D2D operation of the terminal at thefrequency at which the terminal intends to perform the D2D operation anddetermine whether to perform the D2D operation at the frequencyaccording to a state of the selected cell or whether to select the cell.That is, it is preferably determined whether the terminal performs theD2D operation according to a state of not the serving cell of theterminal but the cell which the terminal selects for the D2D operation,in other words, a D2D operating cell. For example, a scenario may beconsidered, in which the serving cell of the terminal is cell #1 offrequency #1, but the D2D operation of the corresponding terminal isperformed in cell #2 of frequency #2. In the scenario, the terminal maydetermine whether to perform the D2D operation by determining cell #2instead of cell #1 is the suitable cell (alternatively, whether theterminal is in the camped normally state) from the viewpoint of theterminal. When the terminal determines whether the terminal is in thecamped normally state with respect to a specific cell in order todetermining whether to perform the D2D operation, the terminaldetermines whether the cell belongs to a separate PLAMN in which the D2Doperation is allowed and when the cell belongs to the separate PLMN, theterminal may determine that the cell satisfies a suitable cell conditionfrom the viewpoint of the PLMN. When the separate PLMN list in which theD2D operation is allowed is configured for the terminal, a PLMN list inwhich the D2D discovery operation is allowed and a PLMN list in whichthe D2D communication operation is allowed may be separately configured.Further, information indicating a PLMN in which the D2D signal isallowed to be received or a PLMN in which the D2D signal is allowed tobe transmitted with respect to the D2D discovery operation or the D2Dcommunication operation may be configured for the terminal.

FIG. 16 illustrates the D2D operation of the terminal according to thefirst method.

Referring to FIG. 16, the terminal in the RRC idle state determineswhether the serving cell is the suitable cell (that is, whether theserving cell is the camped normally state) (S210).

The camped normally state represents a state in which the terminal campson the suitable cell and in the camped normally state, the terminal mayselect and monitor a paging channel according to information giventhrough the system information and perform an evaluation process forcell reselection.

The terminal in the RRC idle state determines whether the serving cellprovides the resource pool information when the serving cell is thesuitable cell, that is, in the normally camped state (S220).

The resource pool information may be provided through the systeminformation provided by the serving cell. A table given below shows oneexample of the system information including the resource poolinformation, which is provided by the serving cell.

TABLE 2 -- ASN1START SystemInformationBlockType19-r12 ::= SEQUENCE {discConfig-r12 SEQUENCE { discRxPool-r12 SL-DiscRxPoolList-r12,discTxPoolCommon-r12 SL-DiscTxPoolList-r12 OPTIONAL, -- Need ORdiscTxPowerInfo-r12 SL-DiscTxPowerInfoList-r12 OPTIONAL, -- Cond TxdiscSyncConfig-r12 SL-SyncConfigList-r12 OPTIONAL -- Need OR } OPTIONAL,-- Need OR discInterFreqList-r12 SL-CarrierFreqInfoList-r12 OPTIONAL, --Need OR lateNonCriticalExtension OCTET STRING OPTIONAL, ... }SL-CarrierFreqInfoList-r12 ::= SEQUENCE (SIZE (1..maxFreq)) OF SL-CarrierFreqInfo-r12 SL-CarrierFreqInfo-r12::= SEQUENCE { carrierFreq-r12ARFCN-ValueEUTRA-r9, plmn-IdentityList-r12 PLMN-IdentityList4-r12OPTIONAL -- Need OP } PLMN-IdentityList4-r12 ::= SEQUENCE (SIZE(1..maxPLMN-r11)) OF PLMN- IdentityInfo2-r12 PLMN-IdentityInfo2-r12 ::=CHOICE { plmn-Index-r9 INTEGER (1..maxPLMN-r11), plmnIdentity-r12PLMN-Identity } -- ASN1STOP

In the table, ‘discInterFreqList’ indicates neighbor frequenciessupported by discovery announcement. ‘discRxPool’ indicates resourceswhich are allowed to receive a discovery signal (e.g., the discoveryannouncement) in the RRC idle state and an RRC connection state.‘discSyncConfig’ indicates a configuration in which the terminal isallowed to transmit or receive synchronization information.‘discTxPoolCommon’ indicates resources (resource pools) in which theterminal is allowed to transmit the discovery signal (e.g., thediscovery announcement) during the RRC idle state. ‘discTxPoolCommon’may be an example of the resource pool information. ‘plmn-IdentityList’indicates lists of PLMN IDs. ‘plmn-Index’ indicates an indexcorresponding to an entry in a ‘plmn-IdentityList’ field of SIB 1systeminformationblock type 1.

When the serving cell of the terminal in the RRC idle state is thesuitable cell and the serving cell provides the resource poolinformation, the terminal transmits the D2D signal within the resourceindicated by the resource pool information (S230).

For example, when the serving cell of the terminal in the RRC idle stateis the suitable cell and ‘discTxPoolCommon’ is included in the systeminformation provided by the serving cell, the terminal may selects theresource pool within the resource pools indicated by the‘discTxPoolCommon’ and thereafter, transmits the discovery announcementby using the selected resource pool.

As a second method, when the terminal camps on the cell, in the casewhere the cell continuously signals information indicating the resourcepool, the terminal may perform the D2D operation by using the signaledresource pool. If the terminal in the RRC idle state is in the any cellselection state, the terminal stops using the resource pool informationwhich the serving cell has signaled most recently.

As a third method, while the terminal caps on the cell, a measurementresult (e.g., RSRP) of the signal received from the cell is equal to ormore than a specific threshold, the terminal performs the D2D operationbased on the information indicting the resource pool signaled by thecell. That is, the second method is different from the first method inthat whether to use the information indicating the resource poolsignaled by the cell is determined according to the measurement resultof the signal received from the cell.

The first and second methods are advantageous in that the methods may beeasily implemented because the operation is simple and the third methodis advantageous in that an area in which the D2D operation may becontrolled by the network is further subdivided. In the presentinvention, all of the aforementioned methods may be used for flexibilityof a policy of the network.

When the first or third method is applied, the terminal may operate asfollows.

While the terminal camps on the cell, the cell may broadcast a thresholdfor a signal strength and resource pool information. In this case, theterminal may compare the measurement result of the signal (e.g., areference signal) received from the cell with the threshold. Only whenthe measurement result is equal to or more than the threshold, theterminal may perform the D2D operation by using the resource poolinformation signaled by the cell.

Alternatively, while the terminal camps on the cell, the cell may notbroadcast the threshold for the signal strength but broadcast only theresource pool information. In this case, the terminal may perform theD2D operation by using the resource pool information signaled by thecell.

Alternatively, while the terminal camps on the cell, the cell may notbroadcast both the threshold for the signal strength and the resourcepool information. In this case, the terminal may not use a radioresource for the D2D operation at a frequency at which the cell ispresent. For example, the terminal is preconfigured with the radioresource which may be used for the D2D operation outside the cellcoverage. However, it is a principle that the terminal may not use thepreconfigured resource after camping on the specific cell and needs toperform the D2D operation according to the control in the specific cell(that is, the network). Therefore, when the specific cell does notprovide the resource for the D2D operation, the terminal may not performthe D2D operation at the frequency of the specific cell.

Hereinafter, a structure of the resource pool information signaled bythe serving cell will be described.

First, points which need to be considered when configuring the resourcepool information are described in terms of the transmission resourcepool and the reception resource pool and thereafter, a detailedstructure of the resource pool information is exemplified.

<D2D Transmission Resource Information>

In cellular communication, the transmission resource is controlled basedon the cell. The terminal needs to be controlled by the serving cell.The principle is preferably similarly applied even to transmitting theD2D signal. Therefore, the terminal may transmit the D2D signal based onand by using resource information corresponding to the serving cell.

The D2D resource information may be resource indication information(e.g., a D2D transmission pool) such as information on a time/frequencywhere a D2D resource is positioned. The D2D resource information may besynchronization information (e.g., a synchronization signal ID andsynchronization signal timing information) for D2D reception or aphysical layer parameter such as a scrambling code applied to a D2Dtransmission signal.

In signaling the resource information which the terminal will use forthe D2D transmission, the network just provides the information on theD2D transmission resource pool corresponding to the serving cell to theterminal and does not announce the transmission resource pool of aneighbor cell to the terminal as the D2D transmission resource.

The terminal that intends to transmit the D2D signal by mode 1 need notknow a set of mode 1 transmission resources, in other words, a mode 1resource pool. Since the network schedules the mode 1 transmissionresource, the terminal may transmit the D2D signal by using the resourceindicated by the network.

However, a receiving terminal that intends to receive the D2D signal bymode 1 within the coverage of the serving cell needs to know the mode 1transmission resource used by a transmitting terminal. Therefore, theresource information including the mode 1 transmission resource of thetransmitting terminal may be announced to the receiving terminal. Inthis case, the resource information including the mode 1 transmissionresource of the transmitting terminal may be announced to the receivingterminal as the mode 1 transmission resource or preferably a mode 1reception resource.

The network needs to particularly announce to the terminal informationindicting a mode 2 transmission resource which may be used fortransmitting the D2D signal by mode 2.

When the network signals the transmission resource pool corresponding tothe serving cell, the network may separately signal the transmissionresource pool like the resource information including the mode 1transmission resource and a mode 2 transmission resource pool.

Alternatively, when the network signals the transmission resource poolcorresponding to the serving cell, the network may signal only the mode2 transmission resource pool as a part of the transmission resource poolcorresponding to the serving cell and signal the mode 1 transmissionresource pool as a part of the reception resource pool.

<D2D Reception Resource Information>

In receiving the D2D signal, the network need not announce receptionresource information separately with respect to each of modes 1 and 2.The reason is that the operation from the viewpoint of the receivingterminal does not vary regardless of which mode of modes 1 and 2 thetransmitting terminal operates.

In this regard, the reception resource information signaled by thenetwork may be commonly applied regardless of modes 1 and 2.

The serving cell and the neighbor cell may configure different D2Dresource information, respectively. In order for the terminal in theserving cell to receive the D2D signal transmitted by using thetransmission resource of the neighbor cell, the terminal needs to knowthe resource information of the neighbor cell. The D2D resourceinformation may be resource indication the information such as theinformation on the time/frequency where the D2D resource is positioned.The D2D resource information may be the synchronization information(e.g., the synchronization signal ID and the synchronization signaltiming information) for the D2D reception or the physical layerparameter such as the scrambling code applied to the D2D transmissionsignal.

One method of two methods given below may be used in order to announcethe D2D resource information of the neighbor cell.

1) A method that announces a common reception resource pool which is aunion of resource pools of each neighbor cell to the terminal.

For example, it is assumed that neighbor cells #1, 2, and 3 are presentand the resources pools of each neighbor cell are #1, 2, and 3. Then,the network announces one common resource pool corresponding to theunion of the resource pools #1, 2, and 3 of the neighbor cells #1, 2,and 3 as the reception resource pool for the terminal. The terminal maymonitor only one reception resource pool in order to receive the D2Dsignal using a neighbor cell transmission resource.

2) A method that separately announces the resources pools of eachneighbor cell to the terminal.

For example, it is assumed that the neighbor cells #1, 2, and 3 arepresent and the resources pools of each neighbor cell are #1, 2, and 3.Then, the network announces a list of resource pools including theresource pools #1, 2, and 3 of the neighbor cells #1, 2, and 3,respectively as the reception resource pool for the terminal. Theterminal needs to monitor the reception resource pool corresponding toeach neighbor cell in order to receive the D2D signal using the neighborcell transmission resource.

3) A method that announces the common reception resource pool which isthe union of the resource pools of each neighbor cell to the terminaland announces the physical layer parameter for each cell.

For example, it is assumed that the neighbor cells #1, 2, and 3 arepresent and the resources pools of each neighbor cell are #1, 2, and 3.Then, the network may announce one common resource pool corresponding tothe union of the resource pools #1, 2, and 3 of the neighbor cells #1,2, and 3 as the reception resource pool for the terminal. The terminalmay monitor only one reception resource pool in order to receive the D2Dsignal using the neighbor cell transmission resource. Togethertherewith, the network signals a physical layer parameter value of theD2D resource used in each cell to the terminal for each neighbor cellapart from the common resource pool. Therefore, the terminal performssynchronization/de-scrambling by applying the physical layer parameterof each neighbor cell in order to receive the D2D signal transmitted byusing the D2D resource of the neighbor cell.

From the viewpoint of the resource pool, when the terminal knows allresource pools of each neighbor cell, the reception resource pools whichtwo methods finally indicate are the same as each other.

However, in order for the receiving terminal positioned at the servingcell to receive the D2D signal transmitted by the transmitting terminalpositioned at the neighbor cell, the receiving terminal needs to knowcell specific parameters such as the physical layer parameters for theneighbor cell, for example the scrambling code or the synchronizationinformation. In this regard, the method that separately announces theresources pools of each neighbor cell to the terminal may be preferable.

When different neighbor cells share same physical layer parameters, theneighbor cells may be grouped and a method that announces the unit ofthe resource pools of the neighbor cells to the terminal as onereception resource pool may be used with respect to the grouped neighborcells.

Meanwhile, information indicating the reception resource pools of theneighbor cells may be provided per cell. For example, the network maysignal the list of the reception resource pools and in the list, eachreception resource pool may be the resource pool of the neighbor cellcorresponding thereto.

The resource pool corresponding to the serving cell may be a union ofthe mode 1 transmission resource pool and the mode 2 transmissionresource pool of the serving cell. In this case, the network need notsignal the resource pool corresponding to the serving cell apart fromthe mode 1 transmission resource pool and the mode 2 transmissionresource pool.

The terminal may configure the reception resource pool corresponding tothe serving cell and configure the reception resource pool from thetransmission resource pool. For example, the terminal may configure theunion of the mode 1 transmission resource pool and the mode 2transmission resource pool as the reception resource pool correspondingto the serving cell.

A table given below shows a structure of the signaled resource poolinformation.

TABLE 3 Information elements Entry # Contents (IEs) Note 1 TX resourcepool RscPool_mode1_tx_s, Transmission resource (of serving cell)RscPool_mode2_tx_s pool for mode 1 and and parameters transmissionresource pool for mode 2 (PHY parameters) Parameters of physical layermay be provided 2 RX resource pool Absent Terminal may regard (ofserving cell) union of and parameters ‘RscPool_mode1_tx_s’ and‘RscPool_mode2_tx_s’ as reception resource pool of serving cell (PHYparameters) Parameters of physical layer may be provided 3 RX resourcepool RscPool_mode1+2_rx_n1 Union of reception of neighbor cell #1resources of modes 1 and parameters and 2 with respect to neighbor cell#1 PHY parameters ID of neighbor cell #1, synchronization information,and the like may be provided 4 RX resource pool RscPool_mode1+2_rx_n2Union of reception of neighbor cell #2 resources of modes 1 andparameters and 2 with respect to neighbor cell #2 PHY parameters ID ofneighbor cell #2, synchronization information, and the like may beprovided . . . RX resource pool RscPool Union of reception of neighborcell#n 1_mode1+2_rx_n . . . resources of modes 1 and parameters and 2with respect to neighbor cell #n PHY parameters ID of neighbor cell #n,synchronization information, and the like may be provided

A table given below shows another example of a structure of the resourcepool information signaled by the serving cell.

TABLE 4 Information elements Entry # Contents (IEs) Note 1 TX resourcepool RscPool_mode2_tx_s Transmission resource (of serving cell) pool formode 2 and parameters (PHY parameters) Parameters of physical layer maybe provided 2 RX resource pool RscPool_mode1_rx_s Reception resource (ofserving cell) pool for mode 1 and parameters Terminal may regard unionof ‘RscPool_mode1_rx_s’ and ‘RscPool_mode2_tx_s’ as reception resourcepool of serving cell (PHY parameters) Parameters of physical layer maybe provided 3 RX resource pool RscPool_mode1+2_rx_n1 Union of receptionof neighbor cell #1 resources of modes 1 and parameters and 2 withrespect to neighbor cell #1 PHY parameters ID of neighbor cell #1,synchronization information, and the like may be provided 4 RX resourcepool RscPool_mode1+2_rx_n2 Union of reception of neighbor cell #2resources of modes 1 and parameters and 2 with respect to neighbor cell#2 PHY parameters ID of neighbor cell #2, synchronization information,and the like may be provided . . . RX resource pool RscPool Union ofreception of neighbor cell#n 1_mode1+2_rx_n . . . resources of modes 1and parameters and 2 with respect to neighbor cell #n PHY parameters IDof neighbor cell #n, synchronization information, and the like may beprovided

Meanwhile, the network and the terminal transmit and receive a D2Dreception assist information request and D2D reception assistinformation in response thereto to more efficiently apply the resourcepool information.

FIG. 17 illustrates a D2D operating method of a terminal according to anembodiment of the present invention.

Referring to FIG. 17, the terminal receives the resource poolinformation from the network (S401).

In Tables 3 and 4 given above, the structure and the configuration ofthe resource pool information have been described. The resource poolinformation may announce the transmission resource pool of the servingcell and the reception resource pool of at least one neighbor cell. Thatis, the resource pool information may indicate a plurality of resourcepools which the terminal needs to monitor.

The terminal requests D2D reception assist information to the network(S402). In other words, the terminal transmits the D2D reception assistinformation request to the network.

The terminal may announce that the terminal intends to receive the D2Dsignal (D2D message) to the network. The terminal transmits a separateRRC message, and the like to the network to announce that the terminalintends to receive the D2D signal (D2D message). As such, when theterminal intends to receive the D2D signal, the terminal may request theD2D reception assist information. Alternatively, the terminal mayrequest the D2D reception assist information through a separateprocedure.

When the terminal requests the D2D reception assist information to thenetwork, the terminal may announce to the network information on the D2Dsignal which the terminal intends to receive.

For example, the terminal may announce a transmission range of the D2Dsignal which the terminal intends to receive to the network. As oneexample, the terminal may announce that the terminal intends to receivethe D2D signal transmitted within 500 m to the network. Alternatively,the terminal may announce a transmission group of the D2D signal whichthe terminal intends to receive to the network. As one example, theterminal may announce a transmission group identifier (Group ID) of theD2D signal which the terminal intends to receive to the network.Alternatively, the terminal may announce the transmitting terminal ofthe D2D signal which the terminal intends to receive to the network. Asone example, the terminal may announce a transmitting terminalidentifier (UE ID) of the D2D signal which the terminal intends toreceive to the network.

Meanwhile, in order to identify from which terminal the network receivesthe D2D reception assist information request, the terminal may make thesame identifier as the ID thereof be included or masked in the D2Dreception assist information request at the time of requesting the D2Dreception assist information.

When the network receives the D2D reception assist information requestfrom the terminal, the network transmits the D2D reception assistinformation to the terminal (S403).

The D2D reception assist information may include information requiredwhen the terminal monitors the D2D signal. Based on the information onthe D2D signal which the terminal intends to receive, correspondinginformation may be included in the D2D reception assist information.That is, the D2D reception assist information may include information toreduce a range of a resource pool which the terminal needs to monitoramong a plurality of resource pools.

For example, the D2D reception assist information may announce one ormore reference cells which the terminal needs to monitor in order toreceive the D2D signal. The reference cell may be associated with one ormore reception resource pools. When the terminal receives reference cellinformation through the D2D reception assist information, the terminalmay monitor the reference pool associated with the reference cell.

Alternatively, the D2D reception assist information may indicate to theterminal one or more resource pools which the terminal needs to monitorin order to receive the D2D signal. The resource pool indicated by theterminal may be identified through a resource pool configuration or anidentifier indicating a specific resource pool among a plurality ofresource pools previously signaled to the terminal. The terminal mayperform monitoring for receiving the D2D signal by using the indicatedresource pool. That is, the D2D reception assist information mayindicate the specific resource pool among the plurality of resourcepools, and as a result, the range of the resource pool which theterminal needs to monitor may be reduced.

The terminal receives (monitors) the D2D signal by using the receptionresource pool indicated by the D2D reception assist information (S404).

For example, it is assumed that resource pool information on the servingcell and three neighbor cells (neighbor cells #1, 2, and 3) is providedfor each cell. It is assumed that when a specific terminal intends toreceive the D2D signal only from terminals having a specific group ID,current locations of the terminals having the specific group ID may notbe known.

When there is no D2D reception assist information, the specific terminalmay need to monitor all unions of the resource pools for each of theserving cell and three neighbor cells in order to receive the D2D signaltransmitted by the terminals having the specific group ID.

On the contrary, the specific group ID is announced to the network atthe time of requesting the D2D reception assist information and thenetwork determines the locations of the terminals having the specificgroup ID to announce to the specific terminal that the terminals arepositioned at a specific neighbor cell (e.g., neighbor cell #2). Then,the specific terminal may monitor only the resource pool of neighborcell #2.

FIG. 18 illustrates a D2D operating method of a terminal according to anembodiment of the present invention.

Referring to FIG. 18, terminal 2 transmits a D2D discovery signal toterminal 1 (S501). In this case, terminal 2 may transmit even an IDthereof together.

Terminal 1 receives resource pool information from the network (S502).

Terminal 1 requests D2D reception assist information to the network(S503). The D2D reception assist information request may include the IDof terminal 2.

After the network receives the D2D reception assist information requestfrom terminal 1, the network transmits the D2D reception assistinformation to terminal 1 (S504). The D2D reception assist informationmay include reception resource pool information of terminal 2.

Terminal 1 receives (monitors) a D2D communication signal by using thereception resource pool indicated by the D2D reception assistinformation (S505).

For example, terminal 1 may receive the D2D discovery signal transmittedby terminal 2, but specific reception pool information may be requiredin order to receive the D2D communication signal transmitted by terminal2. In this case, terminal may announce to a base station (network) theID of terminal 2 included in the D2D discovery signal transmitted byterminal 2. The base station may indicate to terminal 1 the resourcepool required for receiving the D2D communication signal of terminal 2.Terminal 1 monitors the D2D communication signal by using the indicatedresource pool.

FIG. 19 is a block diagram illustrating a terminal in which anembodiment of the present invention is implemented.

Referring to FIG. 19, a terminal 1100 includes a processor 1100, amemory 1120, and a radio frequency (RF) unit 1130. The processor 1110implements a function, a process, and/or a method which are proposed.For example, the processor 1110 determines whether a serving cell of theterminal is a suitable cell and determines whether the serving cellprovides resource pool information. When the serving cell is thesuitable cell and the serving cell provides the resource poolinformation, the processor 1110 transmits a D2D signal within a resourceindicated by the resource pool information. Herein, the terminal 1100may be in an RRC idle state.

The RF unit 1130 is connected to the processor 1110 and sends andreceives radio signals.

The processor may include Application-Specific Integrated Circuits(ASICs), other chipsets, logic circuits, and/or data processors. Thememory may include Read-Only Memory (ROM), Random Access Memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include a baseband circuit for processing a radiosignal. When the above-described embodiment is implemented in software,the above-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memory and executed by the processor. The memory may be disposed tothe processor internally or externally and connected to the processorusing a variety of well-known means.

What is claimed is:
 1. A method for a device-to-device (D2D) operation in a wireless communication system, the method performed by a user equipment (UE) in a radio resource control (RRC) idle state and comprising: determining whether the UE is in a camped normally state on a cell, wherein if the cell is a suitable cell among an acceptable cell, a suitable cell, a barred cell and a reserved cell, then the UE is determined as being in the camped normally state, and wherein the acceptable cell is a cell on which the UE camps to obtain limited service, the suitable cell is a cell on which the UE camps to obtain normal service, the barred cell is a cell on which the UE is not allowed to camp, and the reserved cell is a cell reserved by system information; determining whether the cell provides resource pool information; and transmitting a D2D signal within a resource indicated by the resource pool information when the UE is in the camped normally state on the cell and when the cell provides the resource pool information.
 2. The method of claim 1, wherein the D2D signal is a D2D discovery signal.
 3. The method of claim 1, wherein the resource pool information is provided through system information.
 4. The method of claim 1, wherein the resource pool information indicates a plurality of resource pools.
 5. The method of claim 4, wherein a resource pool is selected among the plurality of resource pools, and the D2D signal is transmitted by using the selected resource pool.
 6. A user equipment (UE) comprising: a radio frequency (RF) unit transmitting and receiving a radio signal; and a processor operated in association with the RF unit, wherein the processor determines whether the UE is in a camped normally state on a cell, wherein if the cell is a suitable cell among an acceptable cell, a suitable cell, a barred cell and a reserved cell, then the UE is determined as being in the camped normally state, and wherein the acceptable cell is a cell on which the UE camps to obtain limited service, the suitable cell is a cell on which the UE camps to obtain normal service, the barred cell is a cell on which the UE is not allowed to camp, and the reserved cell is a cell reserved by system information, determines whether the cell provides resource pool information, and transmits a D2D signal within a resource indicated by the resource pool information when the UE is in the camped normally state on the cell and when the cell provides the resource pool information.
 7. The UE of claim 6, wherein the D2D signal is a D2D discovery signal.
 8. The UE of claim 6, wherein the resource pool information is provided through system information.
 9. The UE of claim 6, wherein the resource pool information indicates a plurality of resource pools.
 10. The UE of claim 9, wherein the processor selects a resource pool among the plurality of resource pools, and transmits the D2D signal by using the selected resource pool.
 11. The UE of claim 6, wherein the UE is in a radio resource control (RRC) idle state. 